Presumably because Marvel had just recovered from their nadir of 17-page comics a few months before, this issue of FF had a 17-page lead story and room for a few more pages. They responded to that with some interesting Kirby content, including the first ever printing of the originally planned cover to FF #3, which is a pretty cool little piece of history. A nice little action shot from the interior story and a demonstration of the powers of the members of the team in the little inset vignettes, while the actual published cover shows off the new costumes better. It was probably a good switch to make, especially at this early issues, setting the book apart from the monster comics which Marvel was publishing.
But they had more pages to fill than that, so we also get the “Feature Page” on Mister Fantastic and what his powers are capable of from FF #16, a pin-up of the Invisible Girl in her section of the Fantasi-Car from FF #10 and two feature pages explaining the Torch’s powers from FF #8 and #9 (including his study of weather patterns to avoid rain). But oddly none of the available pin-up pages of the Thing were used. Poor Benjy…
The pin-up/feature pages are inked by Dick Ayers. The unused cover is also credited to Ayers here, but to Sol Brodsky in the Kirby Checklist. I think Brodsky seems more likely, though the printing here makes it hard to say with any confidence.